The maximum block size is defined in
src/consensus/consensus.h.
/** The maximum allowed weight for a block, see BIP 141 (network rule) */
static const unsigned int MAX_BLOCK_WEIGHT = 4000000;
If you search for
WITNESS_SCALE_FACTOR (which is also defined in the above file as 4), you'll notice it also appears in other parts where there are bytes that need to be measured. In other words there isn't a parameter equal to 1 MB, there's just the above, and the code takes into account the fact that 1 byte is equal with 4 weight units.